The present invention disclosed herein relates to a switch used to control the path of a Radio Frequency (RF) signal in a Microwave Frequency (MF) component and system, and more particularly, to a Single Pole Double Throw (SPDT) switch operating in the RF band such as the MF band.
Due to the development of the wireless communication industry, RF components such as switches, high-output amplifiers, oscillators and low-noise amplifiers, which are fabricated using Integrated Circuit (IC) technology, are being developed in various application fields such as mobile phones and wireless Local Area Networks (LANs). With the development of the wireless communication industry, available frequency bands are increasing and RF components, capable of operating at frequencies higher than the X band, are being required.
Typical SPDT switches are implemented using Field Effect Transistor (FET)-based elements. In designing SPDT switches, large-area FET elements are used as serial switching elements to reduce the insertion loss and small-area FET elements are used as parallel switching elements, connected in parallel to serial switching elements, to improve the isolation characteristics.
Research for improvement of the SPDT switch performance is being conducted variously according to application fields. For example, a circuit structure for an SPDT switch using a parallel resonator is disclosed in Korean Patent Publication No 10-2004-91182. The Korean Patent Publication No 10-2004-91182 discloses a technology for eliminating parasitic components to increase the isolation and decrease the insertion loss.
Also, a structure of adding an impedance conversion circuit between switching elements is disclosed in Korean Patent Publication No. 10-2006-94005. The Korean Patent Publication No. 10-2006-94005 discloses a circuit technology for securely shorting/opening the on/off impedance of a switching to provide a high isolation and a low insertion loss. However, the switch of the Korean Patent Publication No. 10-2006-94005 has narrow-band characteristics because it basically includes a filter structure. Thus, the switch of the Korean Patent Publication No. 10-2006-94005 is limited in terms of application field and is difficult to use in wider bands.
FIG. 1 is a circuit diagram of a typical Complementary Metal Oxide Semiconductor (CMOS) SPDT switch.
Referring to FIG. 1, the typical CMOS SPDT switch includes a serial switching unit and a switching isolation unit. The serial switching unit includes a first CMOS switch 104 and a second CMOS switch 105. The switching isolation unit includes a first N-type MOS (NMOS) transistor 106 and a second NMOS transistor 107. When the serial switching unit operates, the switching isolation unit causes one (i.e., an unselected output terminal) of first and second output terminals P2 and P3 to be electrically isolated from a common input terminal P1.
The first CMOS switch 104 is connected between the first output terminal P2 and the common input terminal P1, and the second CMOS switch 105 is connected between the second output terminal P3 and the common input terminal P1. According to the levels of voltages applied respectively to first and second control terminals VC1 and VC2, a switching path is formed between the common input terminal P1 and the first output terminal P2 or between the common input terminal P1 and the second output terminal P3. For example, when a switch-on voltage is applied to the first control terminal VC1 and a switch-off voltage is applied to the second control terminal VC2, the first CMOS switch 104 is turned on and the second CMOS switch 105 is turned off. In this case, the first NMOS transistor 106 connected to the first output terminal P2 is turned off, and the second NMOS transistor 107 connected to the second output terminal P3 is turned on. Accordingly, an RF signal applied to the common input terminal P1 is outputted through the first CMOS switch 104 to the first output terminal P2. Herein, the first/second NMOS transistor 106/107 connected in parallel to the first/second output terminal P2/P3 serves as a shunt element used to increase the isolation of the switch circuit.
Since the typical CMOS SPDT switch uses the CMOS switches as switching elements, it provides easy switching control and has little power consumption. However, as the signal operating frequency increases, the insertion loss increases and the isolation characteristic of the typical CMOS SPDT switch decreases, particularly by the parasitic capacitance.
FIG. 2 is a simplified circuit diagram and an equivalent circuit diagram of the CMOS switch of FIG. 1, which illustrates a simplified circuit and an equivalent circuit of a CMOS element used as a switching element in an RF/MF band.
Referring to FIG. 2, a MOS transistor 204 switches between a first terminal 202 and a second terminal 203 according to the level of a voltage applied to a control terminal 201, which corresponds to one of the CMOS switches 104 and 105 of FIG. 1. As illustrated in the equivalent circuit diagram, an on/off operation of the MOS transistor 204 may be represented by the size change of a variable resistor VR1. Also, since the CMOS switching element always has a capacitance component, it includes an equivalent capacitor C1 and a junction capacitance at a diode D2. As the operating frequency increases, such a parasitic capacitance component increasingly affects the switching operation, thus decreasing the switching isolation in the RF/MF band.
What is therefore required is a technology for fabricating a switch that has a smaller and simpler structure, provides a lower insertion loss and a higher isolation and has good characteristics even in a wide frequency range.